This invention relates to an apparatus for detecting defects of a pattern with directional characters such as IC photo masks, and more specifically to an apparatus for detecting defects employing a spatial filter for filtering optically coherent light.
A detector for detecting defects of patterns with a directional character such as IC photo masks as well as positional information thereof by applying incoherent light and coherent light to the pattern on the same optical axis has already been proposed by the inventors hereof (U.S. Pat. No. 3,972,616 filed on June 27, 1975; patented on Aug. 3, 1976). The device disclosed in the said U.S. Pat. No. 3,972,616 is an available means for detecting the position of a defect on an IC mask. Because of the shape of the spatial filter as a component of the device of the invention, there still remain such problems as the fact that it is difficult to increase the S/N ratio of the brightness (S) of a defect image to the noise (N) or the light intensities of pattern images in the output plane other than the defects to an adequate degree as well as a shortcoming that the noise level may vary substantially with the change of the subject pattern in size.